This invention relates to apparatus for degassing, submerging, agitating and pumping molten metal. Particularly, this invention relates to mechanical apparatus for moving or pumping molten metal such as aluminum, zinc or magnesium. More particularly, this invention is related to a drive for such apparatus in which a motor is positioned above a molten metal bath and rotates a vertical shaft. The lower end of the shaft drives an impeller or a rotor to impart motion to the molten metal. The invention finds similar application in the construction of the post which supports the motor.
In the processing of molten metals, it is often necessary to pump molten metal from one place to another. When it is desired to remove metal from a vessel, a so called transfer pump is used. When it is desired to circulate molten metal within a vessel, a so called circulation pump is used. When it is desired to purify molten metal disposed within a vessel, a so called gas injection pump is used. In each of these pumps, a rotatable impeller is submerged, typically within a pumping chamber, in the molten metal bath contained in the vessel. Additionally, the motor is suspended on a superstructure over the bath by posts connected to the base. Rotation of the impeller within the pumping chamber forces the molten metal as desired in a direction permitted by the pumping chamber design. The present invention relates to an improved design for each of these types of pumps.
Mechanical pumps for moving molten metal in a bath historically have a relatively short life because of the destructive effects of the molten metal environment on the material used to construct the pump. Moreover, most materials capable of long term operation in a molten metal bath have relatively poor strength which can result in mechanical failure. In this regard, the industry has typically relied on graphite, a material with adequate strength, temperature resistance and chemical resistance, to function for an acceptable period of time in the harsh molten metal environment.
While graphite is currently the most commonly used material, it presents certain difficulties to pump manufacturers. Particularly, mechanical pumps usually require a graphite pump housing submerged in the molten metal. However, the housing is somewhat buoyant in the metal bath because the graphite has a lower density than the metal. In order to prevent the pump housing from rising in the metal and to prevent unwanted lateral movement of the base, a series of vertical legs are positioned between the pump housing and an overhead structure which acts simultaneously to support the drive motor and locate the base. In addition to functioning as the intermediate member in the above roles, the legs, or posts as they are also called, must be strong enough to withstand the tensile stress created during installation and removal of the pump in the molten metal bath.
Similarly, the shaft connecting the impeller and the motor is constructed of graphite. Often, this shaft component experiences significant stress when occluding matter in the metal bath is encountered and sometimes trapped against the housing. Since graphite does not possess as high a strength as would be desired, it would be helpful to reinforce the leg and shaft components of the pump.
In addition, graphite can be difficult to work with because different stock may have different thermal expansion rates and/or different grain orientation. This may result in a post and base having divergent and conflicting thermal expansion rates in the molten metal environment. This problem is compounded by the fact that pump construction has historically required cementing the graphite post into a hole in the graphite base. This design provides no tolerance between the components to accommodate divergent thermal expansion. Unfortunately, this can lead to cracking of the base or the post. Accordingly, it would be desirable to have a molten metal pump wherein the mating of a post and a base is achieved in a manner which accommodates divergent thermal expansion tendencies.
The present invention is equally applicable to a variety of other apparatus used in processing molten metal. Moreover, in addition to pumps, molten metal scrap melting (i.e. submergence), degassing, and agitation equipment, typically rely on the rotation of an impeller/rotor submerged by a vertical shaft in a bath of molten metal. More specifically, a submergence device is used to help melt recycle materials. A major concern of the secondary metal industry is a generation of oxides and gasses which become entrained or dissolved into the molten metal during the melting of scrap metal. These impurities decrease the quality and value of the scrap metal which is ultimately marketable as end product. Accordingly, a degassing device is often used to remove these impurities. In the degasser, a hollow shaft is typically provided to facilitate the injection of gas down the shaft and out through the bores in an impeller/shaft rotor. Typically, the introduced gasses will chemically release the unwanted materials to form a precipitate or dross that can be separated from the remainder of the molten metal bath.
An example of a submergence device is described in U.S. Pat. No. 4,598,899, herein incorporated by reference. An exemplary degassing apparatus is described in U.S. Pat. No. 4,898,367, herein incorporated by reference. In both devices, a vertically oriented shaft having a impeller/rotor disposed at one end in the molten metal bath is employed. Similar problems arise in these apparatus wherein the components are usually constructed of graphite, and would benefit from an increase in strength.
Accordingly, it is a primary advantage of this invention to provide a new and improved molten metal pump.
It is a further advantage of this invention is to provide a new and improved post for a molten metal pump.
Another advantage of this invention is to provide a new and improved shaft for a molten metal pump, degasser or submergence device.
Yet another advantage of the subject invention is to provide a new and improved mechanism for joining of a pump post to a pump base.
A still further advantage of this invention is to provide a molten metal pump post or shaft having improved strength.
Another advantage of this invention is to provide a self-aligning post without a requirement for a cement joint.
Additional advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practicing the invention. The advantages of this invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing advantages in accordance with the purpose of the invention, as embodied and broadly described herein, the molten metal pump of this invention comprises a pumping member (such as an impeller or rotor), at least partially enclosed within a housing. A power device is seated on a support above the housing and pumping member. A shaft connects the power device and the pumping member to provide rotation thereof. At least one, and preferably two to four posts, suspend the housing from the support. One or both of the post or shaft is comprised of an elongated rod surrounded by a heat resistant outer member. In the post embodiment, the rod includes a first end attached to the support (directly or via a coupling) and a second end disposed within a cavity in the housing. Alternatively, the rod can be used strictly for compressing the outer member, which is coupled to the support. In the shaft embodiment, the rod includes a first end secured to the power device (directly or via a coupling) and a second end disposed within a cavity in the pumping member. It is also noted that the shaft embodiment is further suited to use in submergence, degassing and agitation devices.
Preferably, the outer member is comprised of a graphite, refractory, or ceramic material and the housing is comprised of graphite. Preferably, the rod will be comprised of a heat resistant alloy.
In a particularly preferred form of the post embodiment, the rod is biased by a spring. Preferably, the outer member abuts a bottom surface of the support (or an intermediate coupling) and a top surface of the housing and the biasing force of the spring creates a compressive force on the outer member.
In a particularly preferred form of the invention, the outer member is comprised of a plurality of generally cylindrically shaped units, aligned along their longitudinal axis. The rod runs down a central bore of each unit to provide a stacked arrangement. Preferably, the lower most unit will include a circumferential protrusion shaped to mate with a recess formed in the top surface of the housing to create a fluid tight seal.